The present invention relates to an electronic circuit which is formed on an insulating substrate and has a thin semiconductor layer of silicon, for example, forming thin-film transistors, the thin semiconductor layer being required to be connected with conductive interconnects.
Conventional thin-film devices such as insulated-gate FETs use a thin semiconductor film of silicon as an active layer. This layer is about 1500 xc3x85 thick. Therefore, where electrodes should be formed on this thin semiconductor film, satisfactory contacts can be made by bringing a metal such as aluminum into direct and intimate contact with the film, in the same way as in the prior art IC fabrication techniques. In these contacts, a silicide such as aluminum silicide is usually formed by a chemical reaction between the aluminum and the semiconductor component such as silicon. Since the semiconductor layer is sufficiently thicker than the silicide layer, no problems take place.
However, researches conducted recently have demonstrated that if the thickness of the active layer is decreased below 1500 xc3x85, for example, between about 100 to 750 xc3x85, then the characteristics of the TFTs are improved. Where electrodes should be formed on such a thin semiconductor layer, or an active layer, it has not been possible to make good contacts by the prior art techniques, because the thickness of the silicide layer grows almost up to the thickness of the semiconductor layer, thus severely deteriorating the electrical characteristics of the contacts. When a stress such as a voltage is kept applied to the contacts for a long time, the contacts deteriorate seriously.
In order to improve the characteristics of the TFTs, thermal treatment effected below 400xc2x0 C., typically 200-350xc2x0 C., within hydrogen ambient is needed after formation of the electrodes on the semiconductor layer. Where the thickness of the semiconductor layer of the TFTs is less than 1500 xc3x85, the thermal processing greatly promotes growth of the silicide, leading to deterioration of the characteristics of the TFTs.
It is an object of the present invention to provide a reliable electronic circuit having a semiconductor layer, conductive interconnects, and good contacts between the semiconductor layer and the interconnects, the contacts being capable of withstanding thermal processing performed at or above 300xc2x0 C.
The present invention resides in an electronic circuit which is formed on an insulating substrate and has a semiconductor layer consisting mainly of silicon, the thickness of the semiconductor layer being less than 1500 xc3x85, preferably between 100 xc3x85 and 750 xc3x85. For example, the invention is applicable to an electronic circuit having TFTs each provided with an active layer having a thickness less than 1500 xc3x85. The effects of the present invention become conspicuous as the thickness of the semiconductor layer decreases.
In a first embodiment of the invention, the above-described semiconductor layer in the form of a thin film is either in intimate contact with the top surface of the insulating substrate as made of glass or formed over this substrate via some insulating film. A first layer consisting principally of titanium and nitrogen is partially or totally in intimate contact with the semiconductor layer. A second layer consisting principally of aluminum is formed on the top surface of the first layer. The first and second layers are photolithographically patterned into conductive interconnects. The bottom surface of the second layer is substantially totally in intimate contact with the first layer. It is possible to form a third layer consisting mainly of titanium and nitrogen on the second layer.
In another embodiment of the invention, the above-described semiconductor layer in the form of a thin film is either in intimate contact with the insulating substrate as made of glass or formed over this substrate via some insulating film. A first layer containing both titanium and silicon is partially or totally in intimate contact with the semiconductor layer. A second layer consisting chiefly of titanium and nitrogen is in intimate contact with the top surface of the first layer. A third layer consisting principally of aluminum is formed on the top surface of the second layer. The first through third layers are photolithographically patterned into conductive interconnects. Of course, other layer may be formed on the third layer.
In a further embodiment of the invention, the above-described semiconductor layer in the form of a thin film is either in intimate contact with the insulating substrate as made of glass or formed over this substrate via some insulating film. A first layer containing both titanium and silicon as main constituents is partially or totally in intimate contact with the semiconductor layer. A second layer consisting chiefly of titanium and nitrogen is in intimate contact with the top surface of the first layer. A third layer consisting principally of aluminum is formed on the top surface of the second layer. The first through third layers are photolithographically patterned into conductive interconnects. This embodiment is characterized in that the ratio of the titanium to the nitrogen in the first layer is greater than the titanium/nitrogen ratio of the second layer.
In any structure of these embodiments, the portions of the thin semiconductor film with which the first layer is in intimate contact show an N- or P-type conductivity. Preferably, the dose in these portions is 1xc3x971019 to 1xc3x97102/cm20. The impurity may be introduced by a well-known ion implantation method or plasma doping method. Where such impurity ions are accelerated to a high energy and introduced, the dose is preferably between 0.8xc3x971015 and 1xc3x971017/cm2. Also, a laser doping method using laser irradiation within an ambient of an impurity gas may be utilized. This method is described in Japanese Patent application Ser. No. 283981/1991, filed Oct. 4, 1991, and No. 290719/1991, filed Oct. 8, 1991. Preferably, the sheet resistance of these portions is less than 1 kxcexa9/cm2.
Elements which can be added to the semiconductor layer are phosphorus, boron, arsenic, and others. Those portions of the semiconductor layer which are in contact with the conductive interconnects may be parts of doped regions such as the source and drain regions of the TFTs. Preferably, the sheet resistance of the semiconductor layer is less than 500 xcexa9/.
A silicon oxide layer may be in intimate contact with the bottom surface of the thin semiconductor layer. In this structure, the silicon oxide film may contain the same impurity as the impurity contained in the semiconductor layer.
In the first layer of the above-described first embodiment, the ratio of the titanium to the nitrogen contained as main constituents may differ according to the thickness. Besides titanium and nitrogen, other elements such as silicon and oxygen can be contained as main constituents. For example, that portion of the first layer which is close to the semiconductor layer may consist principally of titanium and silicon. That portion of the first layer which is close to the second layer may consist mainly of titanium and nitrogen. For instance, the ratio of nitrogen to titanium may be set close to a stoichiometric ratio (exceeding 0.8). In the intermediate region, the constituents may be made to vary continuously.
Generally, a stoichiometric material (titanium nitride) containing nitrogen and titanium has excellent barrier characteristics and prevents diffusion of aluminum and silicon. However, the material shows a high contact resistance with silicon. Therefore, it is not desired to use such a material directly for formation of contacts. On the other hand, a stoichiometric material (titanium silicide) containing titanium and silicon exhibits a low contact resistance with the semiconductor layer consisting mainly of silicon. This is advantageous to form Ohmic contacts. However, aluminum tends to easily diffuse. For example, the aluminum of the second layer diffuses through the first layer, thus forming aluminum silicide in the semiconductor layer.
The complex layer structure described above has been formed to solve these problems. In particular, that portion which is in contact with the second layer is made of substantially stoichiometric titanium nitride and hence the titanium nitride has excellent barrier characteristics. This prevents the aluminum of the second layer from diffusing into the first layer. The portion in contact with the semiconductor layer is made of substantially stoichiometric titanium silicide. Thus, good Ohmic contacts can be derived.
When a film of titanium silicide is formed, it is not necessary to intentionally add silicon. Titanium reacts with the silicon contained in the semiconductor layer. As a result, titanium silicide is automatically formed. For example, therefore, similar effects can be produced by depositing titanium containing less nitrogen onto the portion close to the semiconductor layer and depositing titanium containing more nitrogen onto the portion close to the second layer.
In either case, when the whole first layer is viewed, it consists mainly of titanium and nitrogen. Preferably, the ratio of nitrogen to titanium in the first layer is 0.5 to 1.2. This material containing titanium and nitrogen as main constituents can make Ohmic contacts with a conductive oxide such as indium tin oxide, zinc oxide, and nickel oxide. Where aluminum and such a conductive oxide together form a junction, a thick layer of aluminum oxide is formed at this junction, and it is impossible to have good contacts. In the prior art techniques, a chromium layer has been formed between aluminum and a conductive oxide. Since the chromium is poisonous, alternative materials have been sought for. Materials used in the present invention and consisting mainly of titanium and nitrogen are excellent also in this respect.
Other objects and features of the invention will appear in the course of the description thereof, which follows.